1. Field of the Invention
The present invention relates to a substrate for a liquid crystal display used in a display section of an electronic apparatus and a liquid crystal display having the same.
2. Description of the Related Art
In general, a liquid crystal display has two substrates each having transparent electrodes and a liquid crystal sandwiched between the substrates. The liquid crystal display is enabled for display in a desired manner by applying a predetermined voltage between the transparent electrodes to drive the liquid crystal and to thereby control light transmittance at each pixel. Recently, there is increasing demand for liquid crystal displays, which has diversified requirements for liquid crystal displays. Above all, improvement of display quality is strongly demanded.
Presently, active matrix type liquid crystal displays having a thin film transistor (TFT) at each pixel as a switching element (TFT-LCDs) have become the main stream of the field. In a TFT-LCD, an interval between the two substrates (cell thickness) is maintained by spherical spacers or bar-shaped spacers made of plastic or glass. Such spacers are normally dispersed on either of the substrates at a spacer dispersing step before the substrates are combined. Thereafter, the two substrates are combined and are pressed from the outside such that the cell thickness is maintained at a value similar to the diameter of the spacers.
However, spacers dispersed within a pixel can cause an alignment defect of the liquid crystal and leakage of light. An alignment defect or leakage of light results in a reduction of contract or glare on the display screen, which degrades display quality. Further, increases in the size of substrates have made it difficult to disperse spacers evenly. When spacers are unevenly dispersed, the cell thickness between the substrates can vary to result in irregularities of luminance. Particularly, in the case of an IPS (In-Plane Switching) or MVA (Multi-domain Vertical Alignment) mode liquid crystal display, variation of luminance in response to variation of the cell thickness is more significant than that in a TN (Twisted Nematic) mode liquid crystal display. Therefore, the cell thickness must be controlled to achieve a higher degree of uniformity of the cell thickness in order to provide display without luminance irregularities. Further, since the trend toward pixels of higher definition has resulted in a reduction of the area of each pixel, the area occupied by spacers relative to that of pixels has increased, and spacers have now more significant influence on display quality.
The recent trend toward substrates in greater sizes and pixels of high definition has resulted in the use of pillar spacers made of a photosensitive resin instead of spherical spacers and bar-shaped spacers. Since pillar spacers are formed at a photolithographic step, they can be disposed in a region that is shielded from light with a black matrix (BM) in an arbitrary disposing density. Therefore, neither alignment defect of the liquid crystal nor leakage of light occurs in pixels, there is no reduction in contrast or occurrence of glare. Further, since pillar spacers can be formed with a uniform thickness (height), control can be performed to obtain a uniform and accurate cell thickness between substrates. Therefore, no luminance irregularity attributable to variation of the cell thickness occurs. As thus described, a liquid crystal display utilizing pillar spacers can achieve display characteristics higher than those of a liquid crystal display utilizing spherical spacers or bar-shaped spacers.
FIG. 13 shows a schematic configuration of a liquid crystal display panel utilizing pillar spacers according to the related art. As shown in FIG. 13, the liquid crystal display panel has a TFT substrate 102, a CF substrate 104 and a liquid crystal sealed between the substrates 102 and 104. The liquid crystal display panel also has a display area 140 in which a plurality of pixels are provided. A BM (picture-frame BM) 149 is formed in a picture-frame area outside the display area 140.
FIG. 14 is an enlarged view of the display area 140 on the CF substrate 104 of the liquid crystal display panel according to the related art. As shown in FIG. 14, a plurality of pixels 146 each constituted by three sub-pixels in blue (B), red (R) and green (G) are arranged at a pitch of 297 μm in each of the horizontal and vertical directions of the figure. The CF substrate 104 is also formed with a BM 148 serving as a light shield for regions between adjoining sub-pixels and storage capacitor portions. In the regions shielded from light by the BM 148, a plurality of pillar spacers 150 are disposed in a disposition density of one per three pixels (nine sub-pixels). The disposition density of the pillar spacers 150 is constant throughout the display area 140.
FIG. 15 shows a schematic configuration of a pillar spacer 150. As shown in FIG. 15, the pillar spacer 150 is in the form of a truncated cone having an upper base surface and a lower base surface both of which are circular. The diameter of the upper base surface of the pillar spacer 150 is 10 μm, and the diameter of the lower base surface is 20 μm. All of the pillar spacers 150 in the display area 140 are formed with a substantially constant size.
The amount of compressive displacement of the pillar spacers 150 is designed such that it is uniform in the display area 140. The amount of compressive displacement must be set an optimum value depending on the design of a cell fabrication step. Specifically, after the two substrates are combined and the gap between the substrates is filled with the liquid crystal at a cell fabrication step, the amount of compressive displacement of the pillar spacers 150 is proportionate to the hardness of the liquid crystal display panel. The amount of compressive displacement of the pillar spacers 150 must be designed such that the pillar spacers 150 have both of flexibility sufficient to follow changes in the volume of the liquid crystal attributable to thermal expansion and thermal shrinkage and hardness sufficient to resist pressures from the outside.
When the pillar spacers 150 are too hard, vacuum regions can be formed because the pillar spacers 150 cannot follow a decrease in the volume of the liquid crystal attributable to thermal shrinkage at a low temperature, and bubbles can be generated in such regions. In general, since a sealing material applied to the picture-frame area is quite hard in comparison to the pillar spacers 150, the picture-frame area and the neighborhood of the periphery of the display area are harder than a central region of the display area, and bubbles are more likely to be generated in those regions. At a liquid crystal filling and sealing step utilizing a dip type vacuum filling process, a panel which has been filled with a liquid crystal is pressed from the outside at a predetermined pressure to eject any extra part of the liquid crystal, whereby the cell thickness is adjusted. However, when the pillar spacers 150 are too hard, the pillar spacers 150 cannot be sufficiently shrunk even if they are pressed at the predetermined pressure. Therefore, when the volume of the liquid crystal increases as a result of thermal expansion at a high temperature, the pillar spacers 150 cannot follow the increase in the volume of the liquid crystal. As a result, the liquid crystal is moved toward a lower part of the panel by gravity, and an irregularity is therefore caused by gravity in that the cell thickness becomes greater in the lower part.
When the pillar spacers 150 are too soft, since they undergo a great amount of displacement and also a great amount of plastic deformation in response to a pressure from the outside, an irregularity of the cell thickness can occur.
As a technique for reducing a liquid crystal filling time, the use of the one drop filling (ODF) method is recently spreading, in which substrate combining and liquid crystal filling is performed at the same time. When the one drop filling method is employed, the cell thickness is determined by the amount of the liquid crystal. Therefore, when the amount of the liquid crystal is too large, a gravity-originated irregularity can occur even if the pillar spacers 150 are soft. Conversely, bubbles can be generated in the picture-frame area at a low temperature when the amount of the liquid crystal is too small.
As thus described, there is a problem in that it is difficult to fabricate a liquid crystal display in which no gravity-originated irregularity occurs and no bubble is generated at a low temperature and which exhibits high resistance to pressures from the outside to achieve high display quality.
Patent Document 1: Japanese Patent Laid-Open No. JP-A-2002-182220
Patent Document 2: Japanese Patent Laid-Open No. JP-A-2003-84289